Soft transition apparatus and method for switching power converters

1. An apparatus comprising: a secondary synchronous rectifier coupled to a secondary side of a power converter comprising a forward switch and a freewheeling switch; and a secondary gate drive controller coupled to a transformer winding of the power converter comprising a secondary synchronous rectifier soft start signal generator configured to generate a plurality of soft start pulses; a pulse width modulation generator configured to generate a forward switch drive signal for the forward switch and a freewheeling switch drive signal for the freewheeling switching based upon a signal across the transformer winding; and a soft transition generator configured to generate a soft start freewheeling switch drive signal by gradually releasing the freewheeling switch drive signal during a soft start process.

2. The apparatus of claim 1 , wherein the soft transition generator generates the soft start freewheeling switch drive signal based upon a product of one of the soft start pulses and a corresponding freewheeling switch drive signal.

3. The apparatus of claim 1 , wherein the transformer winding is a power transformer winding coupled to the forward switch and the freewheeling switch.

4. The apparatus of claim 1 , wherein the transformer winding is an auxiliary transformer winding magnetically coupled to a transformer of the power converter.

5. The apparatus of claim 1 , further comprising a signal shaping circuit coupled between the transformer winding and the secondary gate drive controller.

6. The apparatus of claim 1 , wherein the secondary gate drive controller is configured to: detect a predetermined condition representing a shutdown; reset the secondary synchronous rectifier soft start signal generator; and turn off the forward switch and the freewheeling switch to reduce self-oscillation of secondary side circuits.

7. The apparatus of claim 1 , wherein the secondary gate drive controller is configured such that the power converter monotonically starts up into a pre-biased output.

8. A system comprising: a transformer of a power converter having a primary winding coupled to a dc power source through a primary switch and a secondary winding coupled to an output filter; a secondary synchronous rectifier coupled between the secondary winding and the output filter comprising a forward switch and a freewheeling switch; and a secondary gate drive controller coupled to a transformer winding comprising a secondary synchronous rectifier soft start signal generator configured to generate a plurality of soft start pulses; a pulse width modulation generator configured to generate a forward switch drive signal for the forward switch and a freewheeling switch drive signal for the freewheeling switch based upon a signal across the transformer winding; and a soft transition generator configured to generate a soft start freewheeling switch drive signal by gradually releasing the freewheeling switch drive signal during a soft start process.

9. The system of claim 8 , wherein the power converter is selected from the group consisting of a forward converter, a flyback converter, a half bridge converter, a full bridge converter and a push-pull converter.

10. The system of claim 8 , wherein the power converter monotonically starts up into a pre-biased output by gradually releasing the freewheeling switch drive signal during a soft start process.

11. The system of claim 8 , further comprising an active reset device connected in parallel with the primary switch, wherein the active reset device comprises: an auxiliary switch; and a reset capacitor connected in series with the auxiliary switch.

12. The system of claim 11 , further comprising a primary side controller configured to: generate a primary switch gate drive signal; and generate an auxiliary switch gate drive signal, wherein the auxiliary switch gate drive signal is approximately complementary to the primary switch gate drive signal.

13. The system of claim 8 , wherein the power converter comprises: a first soft start process wherein an output of the power converter gradually increases to a predetermined level; and a second soft start process wherein the freewheeling switch drive signal is gradually released until the output of the power converter reaches the predetermined level.

14. The system of claim 13 , wherein the second soft start process is adjustable by using a resistor coupled to the secondary gate drive controller.

15. A method comprising: detecting a switching pulse from a transforming winding of a power converter; generating a secondary side soft start ramp; generating a plurality of secondary side soft start pulses based upon the secondary side soft start ramp; generating a forward switch gate drive signal; generating a freewheeling switch gate drive signal complementary to the forward switch gate drive signal; and generating a soft start freewheeling switch gate drive signal by gradually releasing the freewheeling switch gate drive signal.

16. The method of claim 15 , further comprising: generating the soft start freewheeling switch drive signal based upon a product of a soft start pulse and a corresponding freewheeling switch drive signal.

17. The method of claim 15 , further comprising: detecting a predetermined condition representing a shutdown; resetting a secondary soft start ramp; and turning off the forward switch and the freewheeling switch to reduce self-oscillation of secondary side circuits.

18. The method of claim 15 , further comprising: detecting the switching pulse from a transforming power winding coupled to the forward switch and the freewheeling switch.

19. The method of claim 15 , further comprising: detecting the switching pulse from a transforming auxiliary winding magnetically coupled to a transformer of the power converter.

20. The method of claim 15 , further comprising: configuring a secondary side gate drive controller such that the power converter monotonically starts up into a pre-biased output.